In the context of the present application, the term THz frequency range designates the range of electromagnetic frequencies between 0.1 to 30 THz. This frequency range corresponds to a range of wavelengths between 10 to 3000 μm or an energy range of 0.4 to 120 meV). The THz frequency range is thus located intermediate between infrared radiation and microwave radiation. THz frequencies offer significant scientific and technological applications with regard to e.g. sensing technology, imaging technology, communication technology, and spectroscopy. For example, recent advances in THz time resolved spectroscopy have allowed the study of conductivity processes in novel organic and inorganic electronic materials with picosecond resolution.
The range of low energy excitations involved allows non-destructive inspection of a large variety of materials. Further, since electromagnetic wavelengths in the THz frequency range are capable of exciting low frequency vibrational modes of condensed phase media as well as vibrational and rotational transitions in molecules, often specific interaction takes place such that a THz frequency absorption spectrum provides a fingerprint of the molecules under examination.
Due to these features of electromagnetic THz radiation, it can for instance advantageously be used to perform spectroscopy of chemical and biological molecules and agents which comprise resonance frequencies being too low to be detected by other known means, such as infrared spectroscopy. It has already been reported that THz frequency range spectroscopy provides interesting aspects with regard to pharmaceutical applications and with regard to security applications, such as detection of explosives and the like.
However, although there are many interesting applications which could advantageously exploit the electromagnetic THz frequency range, only few methods and devices exploiting THz frequencies have been realized thus far. Partly, this results from the fact that sensing at THz frequencies poses new problems which have to be tackled before e.g. THz spectroscopy devices can be realized in a convenient and compact manner. One problem which occurs in the context of THz spectroscopy is that known tabletop THz sources provide relatively low power. This results in limited sensitivity of the known devices. Thus, in order to exploit the promising features offered by the THz frequency range for novel applications, devices will have to be developed which offer higher detection sensitivity and selectivity with regard to such frequencies.
With regard to applications in other frequency ranges of the electromagnetic frequency spectrum, in particular for the optical frequency range, antennas have been developed which exploit plasmonic resonances. For example, it has been shown that suitable plasmonic resonant structures can be created by metal structures. Studies have been performed in which antennas of this type have shown an enhanced interaction with an incoming field and in which theoretical suitability of such antennas for sensing purposes in the optical frequency range has been demonstrated. With regard to wavelengths at 10 μm, sensing has already been demonstrated using metal nanorods, the plasmonic resonance of which can couple to vibrational resonances of the material to detect. However, at THz frequencies metals have a large permittivity value (both in the real part and in the imaginary part) and thus the known concepts are not suited for providing THz frequency range antennas.